1. Field of the Invention
The present invention relates to urea having a low biuret content and, more particularly, to a method of converting crystalline urea into a physical form that has desirable flow properties, resistance to caking and dust formation without any attendant increase in the concentration of biuret.
2. Description of the Prior Art
Urea finds use in a variety of applications. For example, urea is widely used as an agricultural fertilizer. When used as a fertilizer, urea can simply be broadcast on the ground or dissolved in a solution that is sprayed on the foliage of growing plants. In the former case, biuret content is relatively unimportant. However, when used as a foliage spray, the biuret content becomes critical. It is well known that biuret, formed by the condensation of two molecules of urea with the loss of one molecule of ammonia, is noxious to plant life since it exhibits a very active phytotoxic action. Accordingly, to produce a foliar grade urea, it is generally necessary that the urea have a maximum biuret content of 0.25% by weight, more preferably 0.15% by weight or less.
In addition to its use for agricultural purposes, urea has numerous other applications wherein it is required that the biuret content be low. For example, low biuret ureas are necessary in the production of certain synthetic resins and plastics, in pharmaceutical products, in solutions for textile treating and finishing, etc.
High purity crystals of urea can be produced by a process of reacting ammonia and carbon dioxide at high pressures and temperatures to form ammonium carbamate, which, under the reaction conditions, is converted into urea and water. The resulting mother liquor containing urea, ammonium carbamate, ammonia, and water is then treated by various processes to obtain solid urea crystals.
Although urea crystals of high purity and low biuret content can be produced, they are not satisfactory for normal handling, storage, or shipment due to their tendency to agglomerate. Accordingly, most of the industrial and agricultural grades of urea are in the form of prills made in prilling towers or the like well known to those skilled in the art. An inherent problem with the prilling of urea is that the urea is usually heated to or near its melting point with a consequent increase in the biuret content.